The renin-angiotensin system (RAS) is a hormonal cascade of major critical importance to the regulation of blood pressure, fluid and electrolyte balance and kidney function. Angiotensin II (Ang II), the main effector peptide of the RAS, acts at two major receptors, AT1 and AT2. The vast majority of Ang II actions are mediated by the AT1 receptor, including cell proliferation, vasoconstriction and antinatriuresis. Much less is known concerning the functions of the AT2 receptor. Recent studies indicate that the AT2 receptor inhibits cell growth and induces vasodilation opposing the effects of Ang II at AT1 receptors. The role of the AT2 receptors in fluid and electrolyte homeostatsis is unknown. The Principal Investigator has preliminary data demonstrating that the AT2 receptor mediates natriuresis in normal Sprague-Dawley rats and that the heptapeptide derivative of Ang II, des-aspartyl-Ang II (Ang III), is the preferred agonist. The Principal Investigator also has preliminary data strongly suggesting that dopamine D1-receptor stimulation induces increased renal proximal tubule cell apical membrane AT2 receptor expression and that inhibition of AT2 receptors abolishes D1 receptor-induced natriuresis. This project will explore in depth the site and mechanisms of AT2 receptor-induced natriuresis. The project will focus on two specific hypotheses: (1) AT2 receptors in the renal proximal tubule are key mediators of the natriuretic responses to Ang III, AT1 receptor blockade, D1 receptor activation and increased renal perfusion pressure; and (2) AT2 receptor-mediated natriuresis is defective in young pre-hypertensive spontaneously hypertensive rats (SHR). The project will apply a combination of state-of-the-art in vivo and cell and molecular techniques to clarify the role of the AT2 receptor in sodium excretion in hypertension. These studies will help clarify the pathophysiology of human primary hypertension, a disorder affecting one-quarter after adult population in the Western world.